I am in the process of painting my room. I have some big patches on the drywall. I am yet to begin painting that wall. Before i start i wanted to take advice. Is there anything i need to do with those patches so that after paint they are not visible?

or wet sand using a slightly damp rag. If you choose to wet sand, let the area dry before applying the next coat.

For larger holes, you'll have to fill the hole with something solid before you apply compound (don't try to fill the hole with mud). Once the hole is filled, start with a 6" taping knife for the first coat. Next, apply a few thin coats using a 10" taping knife

feathering the compound out to blend into the wall.

When working with drywall compound it's a good idea to use a Hawk, or a mud pan instead of working directly from the bucket. This will insure that you don't end up with little bits of junk in the compound, and that the compound in the bucket won't dry out too much while you're working. They are also good for catching falling mud.

Preparing the wall:

Wipe the wall down with a dry rag, to remove any dust and dirt. Prime the whole wall with a proper primer, like Glidden Gripper.

Any time you patch, it's a good idea to prime the entire wall. If you simply prime the patches, you may be able to see the patches when you're done.

Painting:

Apply 2 coats of the desired paint, and you're done.

If the patches were done properly (smooth and blended appropriately), after primer and two coats of paint you'll never know there was a patch there.

NOTE: Products mentioned in this answer are for example only, I do not specifically endorse or recommend the use of any products mentioned.

Great answer. I am glad i asked the question. I did not know i should apply Primer on the patches. Can you please recommend one as i have no idea what kind of primer i should buy.
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AsdfgOct 12 '11 at 13:42

@Asdfg I have used Glidden Gripper (as mentioned in the answer), and had no complaints. Kilz also makes a good primer.
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Tester101♦Oct 12 '11 at 13:53

You can also try the flashlight trick after priming to see if your patch was smooth. Hold the light a few inches from the wall, parallel to the wall. Any irregularities in the surface will be obvious. There's nothing worse than finishing painting and discovering that you didn't get the mud perfectly smooth.
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DoresoomOct 12 '11 at 14:35

Imperfections are hard to see on a white wall. I learned that adding some tint to primer makes the imperfections obvious BEFORE you paint the whole wall, so they can be patched before finish painting. I add a little of the same paint I am going to use to the primer.